External charger with adjustable alignment indicator

ABSTRACT

Electrical energy is transcutaneously transmitted at a plurality of different frequencies to an implanted medical device. The magnitude of the transmitted electrical energy respectively measured at the plurality of frequencies. One of the frequencies is selected based on the measured magnitude of the electrical energy (e.g., the frequency at which the measured magnitude of the electrical energy is the greatest). A depth level at which the medical device is implanted within the patient is determined based on the selected frequency. For example, the depth level may be determined to be relatively shallow if the selected frequency is relatively high, and relatively deep if the selected frequency is relative low. A charge strength threshold at which a charge strength indicator generates a user-discernible signal can then be set based on the determined depth level.

RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 12/618,637, filed Nov. 13, 2009 (now U.S. Pat. No.9,227,075), which was in turn a non-provisional application claimingpriority to U.S. Provisional Patent Application Ser. No. 61/119,671,filed Dec. 3, 2008. Priority is claimed to these applications, and theyare incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to implantable devices, and moreparticularly, to devices for transcutaneously recharging devicesimplanted within patients.

BACKGROUND OF THE INVENTION

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder sublaxation, etc.The present invention may find applicability in all such applications,although the description that follows will generally focus on the use ofthe invention within a spinal cord stimulation system, such as thatdisclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4,2003 in the name of inventors Paul Meadows et al., which is incorporatedherein by reference in its entirety.

Spinal cord stimulation is a well-accepted clinical method for reducingpain in certain populations of patients. A spinal cord stimulation (SCS)system typically includes an implantable pulse generator and at leaststimulation electrode lead that carries electrodes that are arranged ina desired pattern and spacing to create an electrode array. Individualwires within the electrode lead(s) connect with each electrode in thearray. The electrode lead(s) is typically implanted along the dura ofthe spinal cord, with the electrode lead(s) exiting the spinal column,where it can generally be coupled to one or more electrode leadextensions. The electrode lead extension(s), in turn, are typicallytunneled around the torso of the patient to a subcutaneous pocket wherethe implantable pulse generator is implanted. Alternatively, theelectrode(s) lead may be directly coupled to the implantable pulsegenerator. For examples of other SCS systems and other stimulationsystems, see U.S. Pat. Nos. 3,646,940 and 3,822,708, which are herebyincorporated by reference in their entireties.

Of course, implantable pulse generators are active devices requiringenergy for operation. Oftentimes, it is desirable to recharge animplanted pulse generator via an external charger, so that a surgicalprocedure to replace a power depleted implantable pulse generator can beavoided. To wirelessly convey energy between the external charger andthe implanted pulse generator, the recharger typically includes analternating current (AC) charging coil that supplies energy to a similarcharging coil located in or on the implantable pulse generator. Thissystem is like a loosely coupled inductive transformer where the primarycoil is in the external charger and the secondary coil is in theimplanted pulse generator. The energy received by the charging coillocated on the implantable pulse generator can then be used to directlypower the electronic componentry contained within the pulse generator,or can be stored in a rechargeable battery within the pulse generator,which can then be used to power the electronic componentry on-demand.

To provide efficient power transmission through tissue from the externalcharger to the implanted pulse generator, it is paramount that thecharging coil located in or on the implantable pulse generator bespatially arranged relative to the corresponding AC coil of the externalcharger in a suitable manner. That is, efficient power transmissionthrough the patient's skin from the external charger to the implantablepulse generator via inductive coupling requires constant close alignmentbetween the two devices. Thus, efficiency of the coupling between theexternal charger and implantable pulse generator is largely dependentupon the alignment between the two coils, and in part, determines whatis known as the coupling factor k in a transformer. Achieving a goodcoupling factor is essential for optimizing efficiency of the inductivelink between the external charger and implantable pulse generator. Notonly does good coupling increase the power transferred from the externalcharger to the implantable pulse generator, it minimizes heating in theimplantable pulse generator, and also reduces the power requirements ofthe external charger, which reduces heating of the external charger andminimizes the smaller form factor of the external charger. Propercoupling is also essential for the charging system to function properly,since sufficient coupling is also required for data transfer during thecharging process.

To ensure that such constant close alignment between the externalcharger and implantable pulse generator is achieved, the externalcharger typically includes an alignment indicator that provides a visualor audible signal that can be used by the patient to reposition orreorient the external charger, thereby maintaining or optimizing therate at which the implantable pulse generator is charged. However,achieving proper alignment can be difficult due to the lack ofdifferentiation between a deeply implanted pulse generator that is wellaligned with the external charger and a shallowly implanted pulsegenerator that is poorly aligned, i.e., both scenarios appear the sameto both the external charger and the implantable pulse generator.

One known approach is to use a charge strength indicator on the externalcharger to indicate the extent of the charge rate. For example, a barcharge indicator can be used, such that one bar indicates a relativelylow charge rate, two bars indicate a greater charge rate, three barsindicate an even greater charge rate, and so forth. One downfall ofusing a bar charge connection indicator is that the patient mustcontinually looks at the indicator to ensure an optimal charge rate.

Another approach is to use a misalignment indicator on the externalcharger that signals to the patient with an audible misalignment tonewhenever the charge rate falls below the optimal level. However, thisapproach currently limits the possibility of charging more deeplyimplanted pulse generators at lower rates without inadvertentlytriggering the misalignment tone. Although the alignment zone of theexternal charger could be expanded to prevent such inadvertenttriggering of the misalignment tone, the indicator may not generate themisalignment tone when the charge rate actually is less than optimal.Thus, the patient may charge the implantable pulse generator at asub-optimal rate without ever being warned.

An external charger that combines both a bar charge indicator and amisalignment indicator would still require the patient to monitor thebar charge indicator during charging or endure an audible tone thatinappropriately signals for deeper implantable pulse generators. There,thus, remains a need for an improved method and system for indicatingalignment or misalignment between an external charger and an implantablepulse generator.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present inventions, a method ofcontrolling the charging of a medical device (e.g., a neurostimulationdevice) implanted within a patient is provided. The method comprisestranscutaneously transmitting electrical energy at a plurality ofdifferent frequencies to the implanted medical device. The methodfurther comprises measuring the magnitude of the conveyed electricalenergy respectively at the plurality of frequencies. The magnitude ofthe current within the electrical energy may be measured. For example,if the electrical energy is transcutaneously transmitted from anexternal charger, the electrical current delivered into a primary coilof the external charger may be measured. The magnitude of electricalenergy may be measured at a single relative location or multiplerelative locations between the external charger and the implantedmedical device.

The method further comprises selecting one of the frequencies based onthe measured magnitude of the electrical energy (e.g., the frequency atwhich the measured magnitude of the electrical energy is the greatest),and setting a charge strength threshold at which a charge strengthindicator generates a user-discernible signal based on the selectedfrequency. In one method, the user-discernible signal is binary signal;for example, an audible signal that indicates the occurrence of amisalignment condition or an alignment condition. In one method, thecharge strength threshold is set incrementally higher as the selectedfrequency incrementally decreases, and is set incrementally lower as theselected frequency incrementally increases. Although the broadestaspects of the present inventions should not be so limited, adjustmentof the threshold allows the charge strength indicator to be tailored tothe patient and at the particular depth of the implanted medical device,so that the user-discernible signal is generated at the intended times.

One method further comprises determining a depth level at which themedical device is implanted within the patient based on the selectedfrequency, in which case, the charge strength threshold is set based onthe determined depth level. For example, the depth level may bedetermined to be relatively shallow if the selected frequency isrelatively high, and to be relatively deep if the selected frequency isrelative low. Determining the depth level may comprise estimating adepth value based on the selected frequency. For example, the depthvalue may be estimated by comparing the selected frequency to apre-calibrated frequency/depth curve. The pre-calibrated frequency/depthcurve may be stored in a look-up table, in which case, the depth valuemay be estimated by matching the selected frequency to a frequency valuewithin the look-up table, and obtaining from the look-up table a depthvalue corresponding to the frequency value.

In accordance with a second aspect of the present invention, animplantable medical system is provided. The medical system comprises animplantable medical device (e.g., a neurostimulation device) and anexternal charger configured for transcutaneously conveying electricalenergy to charge the implanted medical device. The medical systemfurther comprises a charge strength indicator (e.g., an audiotransducer) configured for generating a user-discernible signal. In oneembodiment, the indicator is a binary indicator, e.g., an alignmentindicator or a misalignment indicator.

The medical system further comprises a detector configured for measuringthe magnitude of the conveyed electrical energy respectively at theplurality of frequencies, and memory configured for storing a chargestrength threshold at which the charge strength indicator generates theuser-discernible signal. The medical system further comprises aprocessor configured for selecting one of the frequencies based on themeasured magnitude of the electrical energy, and setting the chargestrength threshold based on the selected frequency. The processor canperform these functions in the same manner described above. Thedetector, charge rate indicator, memory, and processor may be containedwithin the external charger.

In accordance with a third aspect of the present invention, an externalcharger for an implantable medical device is provided. The externalcharger comprises a source of electrical power, and an alternatingcurrent (AC) coil configured for transcutaneously conveying electricalenergy from the electrical power source to the implanted medical device.The external charger further comprises a charge strength indicator(e.g., an audio transducer) configured for generating a user-discerniblesignal. In one embodiment, the indicator is a binary indicator, e.g., analignment indicator or a misalignment indicator.

The external charger further comprises a detector configured formeasuring the magnitude of the conveyed electrical energy respectivelyat the plurality of frequencies, and memory configured for storing acharge strength threshold at which the charge strength indicatorgenerates a user-discernible signal. The external charger furthercomprises a processor configured for selecting one of the frequenciesbased on the measured magnitude of the electrical energy, and settingthe charge strength threshold based on the selected frequency. Theprocessor can perform these functions in the same manner describedabove. The external charger may further comprise a portable housingcontaining the electrical power source, AC coil, charge strengthindicator, detector, memory, and processor.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is plan view of one embodiment of a spinal cord stimulation (SCS)system arranged in accordance with the present inventions;

FIG. 2 is a plan view of the SCS system of FIG. 1 in use with a patient;

FIG. 3 is a perspective view of one embodiment of an external chargerused in the SCS system of FIG. 1;

FIG. 4 is a block diagram of the internal components of one embodimentof an external charger and implantable pulse generator used in the SCSsystem of FIG. 1; and

FIG. 5 is an exemplary plot of a frequency/depth curve used by theexternal charger to determine the depth of an implantable pulsegenerator within a patient;

FIG. 6 is a frequency/depth look-up table that can be used to store thefrequency/depth curve of FIG. 5; and

FIG. 7 is a flow diagram of a method used by the external charger tocharge the implantable pulse generator.

DETAILED DESCRIPTION OF THE EMBODIMENTS

At the outset, it is noted that the present invention may be used withan implantable pulse generator (IPG) or similar electrical stimulator,which may be used as a component of numerous different types ofstimulation systems. The description that follows relates to a spinalcord stimulation (SCS) system. However, it is to be understood that thewhile the invention lends itself well to applications in SCS, theinvention, in its broadest aspects, may not be so limited. Rather, theinvention may be used with any type of implantable electrical circuitryused to stimulate tissue. For example, the present invention may be usedas part of a pacemaker, a defibrillator, a cochlear stimulator, aretinal stimulator, a stimulator configured to produce coordinated limbmovement, a cortical and deep brain stimulator, peripheral nervestimulator, or in any other neural stimulator configured to treaturinary incontinence, sleep apnea, shoulder sublaxation, etc.

Turning first to FIG. 1, an exemplary SCS system 10 generally comprisesan implantable neurostimulation lead 12, an implantable pulse generator(IPG) 14, an external (non-implanted) programmer 16, and an external(non-implanted) charger 18.

In the illustrated embodiment, the lead 12 is a percutaneous lead and,to that end, includes a plurality of in-line electrodes 20 carried on aflexible body 22. Alternatively, the lead 12 may take the form of apaddle lead. The IPG 14 is electrically coupled to the lead 12 in orderto direct electrical stimulation energy to each of the electrodes 20.

The IPG 14 includes an outer case formed from an electricallyconductive, biocompatible material, such as titanium and, in someinstances, will function as an electrode. The case forms a hermeticallysealed compartment wherein the electronic and other components areprotected from the body tissue and fluids. For purposes of brevity, theelectronic components of the IPG 14, with the exception of thecomponents needed to facilitate the recharging function (describedbelow), will not be described herein. Details of the IPG 14, includingthe battery, antenna coil, and telemetry and charging circuitry, aredisclosed in U.S. Pat. No. 6,516,227, which is expressly incorporatedherein by reference.

As shown in FIG. 2, the neurostimulation lead 12 is implanted within theepidural space 26 of a patient through the use of a percutaneous needleor other convention technique, so as to be in close proximity to thespinal cord 28. Once in place, the electrodes 20 may be used to supplystimulation energy to the spinal cord 28 or nerve roots. The preferredplacement of the lead 12 is such, that the electrodes 20 are adjacent,i.e., resting upon, the nerve area to be stimulated. Due to the lack ofspace near the location where the lead 12 exits the epidural space 26,the IPG 14 is generally implanted in a surgically-made pocket either inthe abdomen or above the buttocks. The IPG 14 may, of course, also beimplanted in other locations of the patient's body. A lead extension 30may facilitate locating the IPG 14 away from the exit point of the lead12.

Referring back to FIG. 1, the IPG 14 is programmed, or controlled,through the use of the external programmer 18. The external programmer18 is transcutaneously coupled to the IPG 14 through a suitablecommunications link (represented by the arrow 32) that passes throughthe patient's skin 34. Suitable links include, but are not limited toradio frequency (RF) links, inductive links, optical links, and magneticlinks. For purposes of brevity, the electronic components of theexternal programmer 18 will not be described herein. Details of theexternal programmer, including the control circuitry, processingcircuitry, and telemetry circuitry, are disclosed in U.S. Pat. No.6,516,227, which has been previously incorporated herein by reference.

The external charger 18 is transcutaneously coupled to the IPG 14through a suitable link (represented by the arrow 36) that passesthrough the patient's skin 34, thereby coupling power into the IPG 14for the purpose of operating the IPG 14 or replenishing a power source,such as a rechargeable battery (e.g., a Lithium Ion battery), within theIPG 14. In the illustrated embodiment, the link 36 is an inductive link;that is, energy from the external charger 18 is coupled to the batterywithin the IPG 14 via electromagnetic coupling. Once power is induced inthe charging coil in the IPG 14, charge control circuitry within the IPG14 provides the power charging protocol to charge the battery. As willbe described in further detail below, the external charger 18 generatesan audible tone when misaligned with the IPG 14 to alert the user toadjust the positioning of the external charger 18 relative to the IPG14. The external charger 18 is designed to charge the battery of the IPG14 to 80% capacity in two hours, and to 100% in three hours, at implantdepths of up to 2.5 cm. When charging is complete, the external charger18 generates an audible tone to alert the user to decouple the externalcharger 18 from the IPG 14.

Once the IPG 14 has been programmed, and its power source has beencharged or otherwise replenished, the IPG 14 may function as programmedwithout the external programmer 16 being present. While the externalprogrammer 16 and external charger 18 are described herein as twoseparate and distinct units, it should be appreciated that thefunctionality of the external programmer 16 and external charger 18 canbe combined into a single unit. It should be noted that rather than anIPG, the SCS system 10 may alternatively utilize an implantablereceiver-stimulator (not shown) connected to leads 12, 14. In this case,the power source, e.g., a battery, for powering the implanted receiver,as well as control circuitry to command the receiver-stimulator, will becontained in an external controller/charger inductively coupled to thereceiver-stimulator via an electromagnetic link.

Referring now to FIG. 3, the external components of the external charger18 will be described. In this embodiment, the external charger 18 takesthe form of a two-part system comprising a portable charger 50 and acharging base station 52. The charging base station 52 includes an ACplug 54, so that it can be easily plugged into any standard 110 voltalternating current (VAC) or 200 VAC outlet. The charging base station52 further includes an AC/DC transformer 55, which provides a suitableDC voltage (e.g., 5 VDC) to the circuitry within the charging basestation 52.

The portable charger 50 includes a housing 56 for containing circuitry,and in particular, the recharging circuitry and battery (not shown inFIG. 3), which will be discussed in further detail below. The housing 56is shaped and designed in a manner that allows the portable charger 50to be detachably inserted into the charging base station 52, therebyallowing the portable charger 50, itself, to be recharged. Thus, boththe IPG 14 and the portable charger 50 are rechargeable. The portablecharger 50 may be returned to the charging base station 52 between uses.

In the illustrated embodiment, the portable charger 50 includes acharging head 58 connected to the housing 56 by way of a suitableflexible cable 60. The charging head 58 houses the AC coil (not shown inFIG. 3) from which the charging energy is transmitted. The portablecharger 50 further includes a disposable adhesive pouch 62 or Velcro®strip or patch, which may be placed on the patient's skin over thelocation where the IPG 14 is implanted. Thus, the charging head 58 maybe simply slid into the pouch 62, or fastened to the strip or patch, sothat it can be located in proximity to the IPG 14 (e.g., 2-3 cm). In analternative embodiment, the portable charger 50 does not include aseparate charging head, but instead includes a single housing thatcontains the recharging circuitry, battery, and AC coil.

In order for efficient transfer of energy to the IPG 14, it is importantthat the charging head 58 (or more particularly, the AC coil within thehead 58) be properly aligned with the IPG 14. Thus, in the illustratedembodiment, the portable charger 50 includes a bar charge indicator 64located on the housing 56, which provides a visual indication of thestrength of the charge coupling between the charging head 58 and IPG 14in the form of bars. As will be described in further detail below, theportable charger 50 comprises a misalignment indicator in the form of anaudio transducer that provides an audible indication when the charginghead 58 is misaligned relative to the IPG 14. Alternatively, themisalignment indicator can take the form of a vibrating motor. For thepurposes of this specification, both the bar charge indicator 64 andmisalignment indicator can be considered as charge strength indicators.Once proper alignment with the IPG 14 has been achieved, as indicated bythe bar charge indicator 64 or misalignment indicator, the housing 56may simply be taped in place on the patient's skin using removablemedical tape or held in place with a special belt. Typically, chargingof the IPG 14 continues until the battery of the IPG 14 has been chargedto at least 80% of capacity.

Referring to FIG. 4, the recharging elements of the IPG 14 and portablecharger 50 will now be described. It should be noted that the diagram ofFIG. 4 is functional only, and is not intended to be limiting. Those ofskill in the art, given the descriptions presented herein, should beable to readily fashion numerous types of recharging circuits, orequivalent circuits, that carry out the functions indicated anddescribed.

As previously discussed above, the external charger 18 and IPG 14 areshown inductively coupled together through the patient's skin 34 (shownby dotted line) via the inductive link 36 (shown by wavy arrow). Theportable charger 50 includes a battery 66, which in the illustratedembodiment is a rechargeable battery, such as a Lithium Ion battery.Thus, when a recharge is needed, energy (shown by arrow 68) is coupledto the battery 66 via the charging base station 52 in a conventionalmanner. In the illustrated embodiment, the battery 66 is fully chargedin approximately four hours. Once the battery 66 is fully charged, ithas enough energy to fully recharge the battery of the IPG 14. If theportable charger 50 is not used and left on charger base station 52, thebattery 66 will self-discharge at a rate of about 10% per month.Alternatively, the battery 66 may be a replaceable battery.

The portable charger 50 includes a charge controller 70, which serves toconvert the DC power from the AC/DC converter 55 to the proper chargecurrent and voltage for the battery 66, a battery protection circuit 72,which monitors the voltage and current of the battery 66 to ensure safeoperation via operation of FET switches 74, 76, and a fuse 78 thatdisconnects the battery 66 in response to an excessive current conditionthat occurs over an extended period of time. Further details discussingthis control and protection circuitry are described in U.S. Pat. No.6,516,227, which has been previously incorporated herein by reference.

The portable charger 50 further includes a power amplifier 80, and inparticular a radio frequency (RF) amplifier, for converting the DC powerfrom the battery 66 to a large alternating current. The power amplifiermay take the form of an E-class amplifier. The portable charger 50further includes an antenna 82, and in particular a coil, configured fortransmitting the alternating current to the IPG 14 via inductivecoupling. The coil 82 may comprise a 36 turn, single layer, 30 AWGcopper air-core coil having a typical inductance of 45 μH and a DCresistance of about 1.15Ω. The bandwidth the coil 82 is wide enough toaccommodate the full range of frequencies at which the coil 82 isoperated, and thus, need not be turned for a specific resonance. Forpurposes that will be described in further detail below, the poweramplifier 80 is capable of sweeping the frequency of the currentsupplied to the 82. For example, the power amplifier 80 may include avoltage-controlled oscillator (VCO) or a Direct Digital Synthesizer(DDS) (not shown) that is capable of varying the frequency of thecurrent output by the power amplifier 80.

The IPG 14 includes an antenna 84, and in particular a coil, configuredfor receiving the alternating current from the portable charger 50 viathe inductive coupling. The coil 84 may be identical to, and preferablyhas the same resonant frequency as, the coil 82 of the portable charger50. The IPG 14 further comprises rectifier circuitry 86 for convertingthe alternating current back to DC power. The rectifier circuitry 86may, e.g., take the form of a bridge rectifier circuit. The IPG 14further includes a rechargeable battery 88, such as a Lithium Ionbattery, which is charged by the DC power output by the rectifiercircuitry 86. In the illustrated embodiment, the battery 88 can be fullycharged by the portable charger 50 in under four hours (80% charge intwo hours).

The portable charger 50 includes a charge controller 90, which serves toconvert the DC power from the rectifier circuitry 86 to the propercharge current and voltage for the battery 88, a battery protectioncircuit 92, which monitors the voltage and current of the battery 88 toensure safe operation via operation of a FET switch 94, and a fuse 96that disconnects the battery 88 in response to an excessive currentcondition that occurs over an extended period of time. Further detailsdiscussing this control and protection circuitry are described in U.S.Pat. No. 6,516,227, which has been previously incorporated herein byreference.

As briefly discussed above, the portable charger 50 is capable ofindicating when the battery 88 of the IPG 14 is fully charged or almostfully charged, and when the portable charger 50 is aligned/misalignedwith the IPG 14. To this end, the portable charger 50 comprises chargedetection circuitry 98 in the form of a passive telemetry data receiverfor detecting an electrical parameter indicative of the charge rate ofthe IPG 14, and a processor 100 for determining the charging qualitiesof the IPG 14, and in particular, when the IPG 14 is fully charged andwhen the portable charger 50 is aligned/misaligned with the IPG 14,based on the detected electrical parameter. The charge detectioncircuitry 98 may comprise separate components for respectively receivingpassive data from the IPG 14 and for detecting when the portable charger50 is aligned/misaligned with the IPG 14. For purposes of brevity, thesecomponents are described herein as a single component. The portablecharger 50 further comprises memory 102 for storing a charge strengththreshold that the processor 100 uses to determine misalignment (oralternatively, alignment) between the portable charger 50 and IPG 14. Inthe embodiment described below, the charge strength threshold takes theform of a voltage threshold value. The memory 102 also store a computerprogram used by the processor 100 to perform the functions describedbelow.

In addition to the previously described bar charge indicator 64 (shownin FIG. 3), which visually indicates the charge coupling of the IPG 14to the user, the portable charger 50 also includes an indicator 106 inthe form of an audio transducer (speaker), which signals the user withan audible tone when the battery 88 of the IPG 14 is fully charged andwhen the portable charger 50 is misaligned with the IPG 14. Inalternative embodiments, separate indicators can be used to indicate afull charge state and a misalignment condition.

In the illustrated embodiment, the electrical parameter sensed by thecharge detection circuitry 98 is a steady-state voltage having a valueV1 at the coil 82, which is indicative of the charge rate of the IPG 14.That is, the voltage value V1 (which is dictated by the reflectedimpedance from the coil 84 of the IPG 14) is inversely proportional tothe coupling between the coils 82, 84 of the respective portable charger50 and IPG 14, and thus, the charge rate of the IPG 14. Thus, as thereflected impedance and thus the voltage value V1 increases, the chargerate decreases, and as reflected impedance and thus the voltage value V1decreases, the charge rate increases.

The charge detection circuitry 98 also senses the voltage value V1 atthe coil 82 to detect when the IPG 14 has been fully charged. Inparticular, the IPG 14 includes a back telemetry circuit 104, whichdetects charge completion of the battery 88 and modulates the secondaryload of the IPG 14 by changing the rectifier circuitry 86 from afull-wave rectifier into a half-wave rectifier/voltage clamp. Thismodulation, in turn, suddenly increases the reflected impedance into thecoil 82 of the portable charger 50, which suddenly increases the voltagevalue V1 (e.g., a transient or pulsed component appears in the detectedsteady-state voltage) detected by the charge detection circuitry 98.

The processor 100 receives the voltage value V1 from the chargedetection circuitry 98, and based on this value, operates the bar chargeindicator 64 and audio transducer 106 accordingly. In particular, if thevoltage value V1 spikes or suddenly increases, the processor 100determines that the battery 88 of the IPG 14 is fully charged, andprompts the audio transducer 106 (e.g., by sending a signal) to generatean audible tone or series of audible tones (e.g., an ON-OFF beepingsound), thereby alerting the user that the IPG 14 is fully charged.

The processor 100 operates the bar charge indicator 64 to display theproper number of bars in accordance with the charge rate indicated bythe voltage value V1. The processor 100 also compares the voltage valueV1 to the electrical parameter threshold value (in this case, a voltagethreshold value) stored in the memory 102 to determine misalignmentbetween the portable charger 50 and IPG 14. In particular, the processor100 compares the voltage value V1 with the voltage threshold valuestored in the memory 102 to determine whether a misalignment conditionhas occurred (i.e., the measured voltage value V1 exceeds the voltagethreshold value), and operates the audio transducer 106 in a binaryfashion, meaning that it only indicates if a particular condition hasbeen satisfied or not satisfied (i.e., misaligned or not misaligned).

While the modification of a voltage threshold value (or other suitableelectrical parameter threshold value) lends itself well to setting thethreshold of an audible misalignment tone, thereby ensuring that thepatient is alerted only when the portable charger 50 is actuallymisaligned with the IPG 14, in alternative embodiments, the adjustablevoltage threshold value can be used to modify the threshold at which abinary indicator generates a user-discernible signal other than amisalignment signal. For example, the voltage threshold value cancorrespond to an audio transducer that sounds an alignment tone (i.e.,an audible tone that alerts the patient that the portable charger 50 isaligned with the IPG 14), or an indicator that illuminates an alignmentlight (i.e., a visual signal that alerts the patient that the portablecharger 50 is aligned with the IPG 14). In other embodiments, one ormore adjustable charge rate threshold values can be used to modify thethreshold(s) at which non-binary indicators generate user-discerniblesignals. For example, the charge rate threshold value(s) can correspondto a bar indicator, such as, e.g., the bar charge indicator 64, suchthat the thresholds at which the number of bars increases or decreasescan be adjusted.

While the illustrated embodiment has been described as performing thecharge rate indication and processing functions in the portable charger50, it should be appreciated that any of these functions can beperformed in the charger base station 52, or even the IPG 14. If theindication function is performed by the IPG 14, the user-discerniblesignal can take the form of a vibration or a modulated electricalstimulation.

Significantly, the voltage threshold value stored in the memory 102 canbe varied in order to modify the actual charge rate at which amisalignment condition is deemed to occur. Thus, if the IPG 14 isimplanted relatively deep within the patient, the voltage thresholdvalue can be increased, so that the audible misalignment tone does notsound when the charge rate is optimum or otherwise suitable for thatimplant depth. In contrast, if the IPG 14 is implanted relativelyshallow within the patient, the voltage threshold value can bedecreased, so that the audible misalignment tone sounds when the chargerate is not optimum or otherwise suitable for that implant depth. Thus,the audible misalignment tone will only sound when the charge rate issub-optimal for the specific implant depth or orientation. Notably, thecharge strength is inversely proportional the voltage value V1, andtherefore, it can be said that adjustment of the voltage threshold valueis inversely proportional to an adjustment of the charge strengththreshold (i.e., an increase in the voltage value threshold is actuallya decrease in the charge strength threshold, and a decrease in thevoltage value threshold is actually an increase in the charge strengththreshold).

Adjustment of the voltage threshold value can be accomplished byautomatically determining the depth level at which the IPG 14 isimplanted within the patient, with the voltage threshold value beingdecreased if the depth level is determined to be relatively deep, andthe voltage threshold being increased if the depth level is determinedto be relatively shallow. Significantly, in determining the depth levelof the IPG 14, the processor 100 is capable of sweeping the frequency ofthe electrical current output from the power amplifier 80 to the coil 82(e.g., by varying the voltage input to a voltage-controlled oscillator(VCO) within the power amplifier 80). In one embodiment, the processor100 sweeps the frequency of the electrical current input to the coil 82in the range from 115 KHz to 77 KHz at 100 Hz intervals.

The relative distance between the IPG 14 and the external charger 50,and thus, the depth level of the IPG 14 within the patient, can bedetermined by measuring the magnitude of the electrical current inputinto the coil 80 at the respective different frequencies, anddetermining the depth level based on the measured electrical currentmagnitudes. Other electrical parameters, e.g., the steady-state voltagevalue V1, can be measured to determine the depth level of the IPG 14.Such measurements can be performed in the external charger 50 or the IPG14. Notably, if the IPG 14 is relatively far away from the externalcharger 50 (i.e., the depth level of the IPG 14 is relatively deep), themeasured electrical current will peak at a relatively low frequency, andif the IPG 14 is relatively close to the external charger 50 (i.e., thedepth level of the IPG 14 is relatively shallow), the measuredelectrical current will peak at a relatively high frequency. Thus, thedepth level of the IPG 14 may be determined by identifying the frequencyat which the measured electrical current is the greatest.

To this end, the external charger 50 comprises an electrical currentdetector 108 that measures the magnitude of the electrical current inputfrom the power amplifier 80 into the coil 82, and continually outputsthe measured magnitudes to the processor 100 as the frequency of thecurrent is varied. The processor 100 identifies the frequency at whichthe measured electrical current is the greatest, stores this frequencywithin memory 102, and based on this frequency, determines the depthlevel of the IPG 14 (i.e., determines the depth level to be relativelyshallow if the determined frequency is relatively high, and determinesthe depth level to be relatively deep if the determined frequency isrelatively low). In some cases, if the external charger 50 is movedaround during the monitoring process, different frequencies at which themeasured electrical current is the greatest will occur, in which case,the processor 100 will store the highest frequency at which the measuredelectrical current is the greatest, and use that to determine the depthlevel of the IPG 14. Based on the determined depth level, the processor100 will automatically modify the voltage threshold value stored in thememory 102, with the voltage threshold value being set to a relativelyhigh value if the depth level of the IPG 14 is determined to berelatively deep, and the voltage threshold value being set to arelatively low value if the depth level of the IPG 14 is relativelyshallow.

In one embodiment, the processor 100 is configured for estimating thedepth level of the IPG 14 by comparing the frequency at which thegreatest magnitude of the electrical current was measured to apre-calibrated curve of frequencies versus depth, an exemplary one ofwhich is illustrated in FIG. 5. The pre-calibrated frequency/depth curvecan be generated using a variety of means, such as implanting an IPG atvarious known sample depths in tissue and measuring the current inputinto the coil of an external charger while sweeping the frequency rangefor each implant depth. Additional data points may optionally be addedto the pre-calibrated frequency curve depth to increase the depthestimation by interpolating between the measured data points.

The pre-calibrated frequency/depth curve may be stored in the memory 102as a look-up table, as illustrated in FIG. 6. In this case, theprocessor 100 may recall the look-up table and compare the frequencyvalue at which the greatest magnitude of the electrical current wasmeasured to the frequencies values contained within the look-up table.The depth of the IPG 14 can then be estimated as the depth valuecorresponding to the stored frequency value that best matches themeasured frequency value. For example, if the frequency value at whichthe greatest magnitude of the electrical current was measured is 114.5KHz, the depth of the IPG 16 will be estimated to be 49.375 mm.

The processor 100 can then adjust the voltage threshold value stored inmemory 102 to a value corresponding to the estimated depth value, withthe voltage threshold value incrementally increasing as the estimateddepth value incrementally increases, and the voltage threshold valueincrementally decreasing as the estimated depth value incrementallydecreases. Alternatively, the depth values stored in the look-up tablecan be replaced with corresponding voltage threshold values, so that theprocessor 100 can directly obtain the voltage threshold value from thelook-up table without estimating the depth value.

Notably, the resolution of the look-up table, and thus the accuracy ofthe estimated depth, will depend on the intervals of the frequency rangeat which the electrical current magnitude is measured. For example, inthe exemplary case wherein the frequency of the electrical current isswept in the range from 115 KHz to 77 KHz at 100 Hz intervals, thelook-up table will have 381 magnitude values and 381 correspondingfrequency values, and therefore, the number of depth values, and thusvoltage threshold values, from which the processor 100 may select is381.

Having described the structure and function of the charging system, onemethod of recharging the IPG 14 with the external charger 50 will now bedescribed with reference to FIG. 7. First, the external charger 50 isplaced in the general vicinity of the implanted IPG 14 (step 200). Then,the external charger 50 is turned on, at which time it will generate anaudible misalignment tone to indicate a misalignment condition until theexternal charger 50 is sufficiently aligned with the IPG 14 (step 202).Alternatively, the external charger 50 will generate an audiblealignment tone to indicate an alignment condition only when the externalcharger 50 is sufficiently aligned with the IPG 14. The external charger50 will then begin transcutaneously transmitting electrical energy tocharge the implanted IPG, while sweeping the frequency of the electricalcurrent provided to the primary coil 82 of the external charger 50 (step204). While the frequency of the electrical current is swept, themagnitude of the electrical current is measured at the differentfrequencies (step 206).

The external charger 50 may either be maintained at the same relativelocation to the IPG 14 at the site, in which case, a single set ofelectrical current magnitudes will be obtained for the entire frequencyrange, or the external charger 50 may be slowly moved around at thesite, in which case, multiple sets of electrical current magnitudes willbe obtained for the entire frequency range to in order to map the site.In either case, external charger 50 determines the frequency at whichthe magnitude of the electrical current peaks and stores the highestfrequency (step 208). The external charger 50 then estimates the depthof the IPG 14 by accessing the frequency/depth look-up table stored inmemory 102, identifying within the look-up table the frequency valuethat matches the determined highest frequency, and selecting thecorresponding depth value from the look-up table (step 210). Next,voltage threshold value stored in memory 102 is set or adjusted to avalue based on the estimated depth of the IPG 14; with the adjustedvoltage threshold value being incrementally higher as the estimateddepth decreases, and the adjusted voltage threshold value beingincrementally lower as the estimated depth increases (step 212).

After adjustment of the voltage threshold value, the stead steady-statevoltage value V1 at the coil 102 is measured (step 214) and compared tothe voltage threshold value (step 216). If the voltage value V1 exceedsthe voltage threshold value, the audible sound will continue to indicatethe continued presence of the misalignment condition (step 218), and ifthe voltage value V1 does not exceed the voltage threshold value, theaudible sound will be stopped to indicate an alignment condition (step220). Alternatively, if the voltage value V1 exceeds the voltagethreshold value, no audible sound will be generated to indicate thecontinued presence of the misalignment condition, and if the voltagevalue V1 does not exceed the voltage threshold value, the audible soundwill be generated to indicate an alignment condition. Steps 214-220 canthen be repeated without adjusting the voltage threshold value. Theprocess can return to step 204 to readjust the voltage threshold value.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. An external charger for an implantable medicaldevice, comprising: a coil configured to generate a magnetic field toprovide power to the implantable medical device; an indicator configuredto indicate to a user alignment of the external charger to theimplantable medical device in accordance with a threshold; controlcircuitry, wherein the control circuitry is configured to drive the coilat a plurality of different frequencies within a frequency range togenerate the magnetic field; and a detector configured to measure amagnitude of a current of the coil at each of the plurality offrequencies, wherein the control circuitry is further configured to:select a frequency using the measured current magnitudes, and select thethreshold for use by the indicator using the selected frequency.
 2. Theexternal charger of claim 1, wherein the control circuitry is configuredto select the frequency at which the measured current magnitude isgreatest.
 3. The external charger of claim 1, wherein the controlcircuitry comprises a processor.
 4. The external charger of claim 1,wherein the control circuitry further comprises a memory, wherein thememory comprises data correlating frequencies with threshold values. 5.The external charger of claim 4, wherein the control circuitry isconfigured to select the threshold by comparing the selected frequencyto the data correlating frequencies with threshold values.
 6. Theexternal charger of claim 5, wherein comparing the selected frequency tothe data correlating frequencies with threshold values comprisesselecting a threshold value as the threshold using the selectedfrequency.
 7. The external charger of claim 1, further comprising amemory, wherein the selected threshold for use by the indicator isstored in the memory.
 8. The external charger of claim 1, wherein theindicator comprises an audio transducer configured to produce an audiblealignment indication.
 9. The external charger of claim 1, wherein theindicator comprises a visual indicator configured to produce a visualalignment indication.
 10. The external charger of claim 1, wherein theindicator indicates an extent of a charge rate of the implantablemedical device.
 11. The external charger of claim 1, further comprisinga battery for providing power to generate the magnetic field.
 12. Theexternal charger of claim 1, wherein selected threshold increases as theselected frequency decreases, and wherein the selected thresholddecreases as the selected frequency increases.
 13. The external chargerof claim 1, wherein the control circuitry is further configured todetermine a depth at which the implantable medical device is implantedwithin a patient based on the selected frequency.
 14. The externalcharger of claim 13, wherein the determined depth increases as theselected frequency decreases, and wherein the determined depth decreasesas the selected frequency increases.
 15. The external charger of claim13, further comprising a memory, wherein data correlating frequencieswith depth values is stored in the memory.
 16. The external charger ofclaim 1, further comprising a portable housing for the coil, theindicator, the control circuitry, and the detector.
 17. The externalcharger of claim 1, further comprising a portable housing for theindicator, the control circuitry, and the detector, wherein the coil isconnected to the housing by a cable.
 18. The external charger of claim1, wherein the control circuitry comprises a controllable oscillator tosweep the frequency at which the coil is driven.